JavaScript For In
The For In Loop
The JavaScript for in statement loops through the properties of an Object:
Syntax
for (key in object) {
// code block to be executed
}
Example
const person = {fname:"John", lname:"Doe", age:25};
let text = "";
for (let x in person) {
text += person[x];
}
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Example Explained
- The for in loop iterates over a person object
- Each iteration returns a key (x)
- The key is used to access the value of the key
- The value of the key is person[x]
For In Over Arrays
The JavaScript for in statement can also loop over the properties of an Array:
Syntax
for (variable in array) {
code
}
Example
const numbers = [45, 4, 9, 16, 25];
let txt = "";
for (let x in numbers) {
txt += numbers[x];
}
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Do not use for in over an Array if the index order is important.
The index order is implementation-dependent, and array values may not be accessed in the order you expect.
It is better to use a for loop, a for of loop, or Array.forEach() when the order is important.
Array.forEach()
The forEach() method calls a function (a callback function) once for each array element.
Example
const numbers = [45, 4, 9, 16, 25];
let txt = "";
numbers.forEach(myFunction);
function myFunction(value, index, array) {
txt += value;
}
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Note that the function takes 3 arguments:
- The item value
- The item index
- The array itself
The example above uses only the value parameter. It can be rewritten to:
Example
const numbers = [45, 4, 9, 16, 25];
let txt = "";
numbers.forEach(myFunction);
function myFunction(value) {
txt += value;
}
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